The present invention relates to a disk size detecting circuit for detecting the size of a disk set in a video disk player or audio disk player.
A conventional disk size detecting circuit is shown in FIG. 1. In FIG. 1, a rotational speed detecting pulse generator 3 outputs a rotational speed detecting pulse signal having a pulse repetition frequency corresponding to the rotational speed of a motor 2 driving a disk 1. The pulse generaator 3 is designed as follows: a slit is formed in the periphery of a disk which is fixed to the rotary shaft of the motor 2. Light is applied to the periphery of the disk where the slit if formed, and a photoelectric conversion element produces a pulse when light passes through the slit.
The output pulse signal of the rotational speed detecting pulse generator 3 is applied to a frequency detecting circuit 4 forming a rotational speed detecting circuit. In the frequency detecting circuit 4, the output pulse signal of the pulse generator 3 is applied to an F/V (frequency-to-voltage) converter circuit 5. The F/V converter circuit 5 is formed with an integrator circuit so as to provide a voltage corresponding to the repetition frequency of the pulse signal received, that is, the rotational speed of the motor 2. The output voltage of the F/V converter circuit 5 is applied to a comparator where it is compared with a reference voltage V.sub.r. When the speed of the motor 2 reaches a predetermined value, that is, when the output voltage of the F/V converter circuit 5 reaches the reference voltage V.sub.r, the output of the comparator 6 is raised to a high level. The high level signal indicates the fact that the speed of the motor 2 has reached a predetermined value to a discriminating circuit 7, to which the output of a timer 8 is also supplied. In the timer 8, the output clock pulse signal from a reference oscillator 9 is subjected to frequency division to provide a pulse signal having, for instance, a period of 1/100 second. The latter signal is applied as a clock pulse signal input to a counter, the output count data of which indicates, with an accuracy of 1/100 second, the amount of time lapsed following generation of a start signal to start the motor 2. Furthermore, the discriminating circuit 7 is designed so that, upon generation of the predetermined speed detection signal, the output data of the timer 8 and predetermined reference value are subjected to comparison so that, when the output data of the timer 8 is a value less than the predetermined reference value, the output of the discriminating circuit is set to a low level, and, when the output data of the timer 8 is higher than the predetermined reference value, the output is raised to a high level. The output of the discriminating circuit 7 is applied to an output terminal OUT.
In the circuit thus constructed, the moment of inertia of the disk 1, which is proportional to the square of the radius of the disk 1, affects the motor rise time. Thus, if the voltage applied to the motor is constant, the time required for the speed of the motor to reach a predetermined value decreases as the moment of inertia decreases. Therefore, the period of time which elapses from the instant that the start signal used to start the motor 2 is generated until the predetermined speed detection signal is produced changes according to the size (diameter) of the disk 1.
Accordingly, the circuit can be designed so that, when the size of the disk 1 is small, the count value of the timer 8 at the generation of the predetermined speed detection signal is equal to or smaller than a predetermined reference value, and when the size of the disk is large, the count value of the timer 8 at the generation of the predetermined speed detection signal is larger than the predetermined reference value. In this case, the output of the discriminating circuit 7 is set to a low level when the size of the disk 1 is small, and the output of the circuit 7 is raised to a high level when the size of the disk 1 is large. In this manner, the size of the disk can be determined.
In the above-described conventional disk size detecting circuit, the rise time of the motor 2 is, in general, long compared with the repetition frequency of the clock pulse signal. Accordingly, relatively many flip-flop circuits are required to subject the clock pulse signal to frequency division and to form the counter in the timer 8. As a result, the circuit is necessarily large in scale and high in manufacturing cost.